SUMMARY QUESTIONS Flashcards
The following is a sequence of 12 nucleotides within a much larger mRNA molecule: AUG CAU GUU ACU. Following a gene mutation the same 12-nucleotide portion of the mRNA molecule is AUG CUG UUA CUG. Name the type of gene mutation that has occurred. Show your reasoning
A deletion because the 5th nucleotide (A) has been lost. The sequence prior to and after this is the same
Explain why a deletion gene mutation is more likely to result in a change to an organism than a substitution gene mutation
In a deletion, all codons after the deletion are affected. Therefore most amino acids coded for by these codons will be different and the polypeptide will be significantly affected. In a substitution, only a single codon, and therefore a single amino acid, will be affected. The effected on the polypeptide is likely to be less severe
Explain why a mutation that is transcribed onto mRNA may not result in any change to the polypeptide that it codes for
The mutation may result from the substitution of 1 base in the mRNA with another. Although the codon affected will be different, as the genetic code is degenerate, the changed codon may still code for the same amino acid. The Polypeptide will be unchanged and there will be no effect
Errors in transcription occur about 100 000 times more often than errors in DNA replication. Explain why errors in DNA replication can be far more damaging than errors in transcription
These errors may be inherited and may therefore have a permanent affect on the whole organism. Errors in transcription usually affect only specific cells, are temporary and not inherited. They are therefore less damaging
A cell is examined and found to have 27 chromosomes. Is it likely to be haploid or diploid? explain your answer
Haploid because 27 is an odd number. Diploid cells have 2 sets of chromosomes and so their total must be an even number
state 2 ways in which meiosis leads to an increase in genetic variety
- independent segregation of homologous chromosome
- recombination by crossing over
A mule is a cross between a horse (64 chromosomes) and a donkey (62 chromosomes). Mules therefore have 63 chromosomes. From your knowledge of meiosis, suggest why mules are sterile
Gametes are produced by meiosis. in meiosis, homologous chromosomes pair up. With 63 chromosomes precise pairings are impossible. This prevents meiosis and hence gamete production, making them sterile
Calculate the number of possible chromosome combinations produced from the fertilisation of 2 gametes, each of which contains 5 chromosomes (assuming there is no crossing over)
1024 (the haploid number of 5 is the same as the number of homologous chromosomes)
State whether each of the following is likely to increase or decrease genetic diversity:
a) increasing the variety of alleles within a population
b) breeding together closely related cats to develop varieties with longer fur
C) mutation (permanent change to the DNA) of an allele
A) increase
B) decrease
C) increase
Explain how a difference in its DNA might lead to an organism having a different appearance and hence the species showing greater genetic diversity
Different DNA - different codes for amino acids - different amino acids - different protein shape - different protein function (e.g. non-functional)
what is selection
selection is the process by which organisms that are better adapted to their environment survive and breed, while those less well adapted fail to do so
Distinguish between directional selection and stabilising selection
DIRECTIONAL SELECTION
- favours/selects phenotypes at one extreme of a population
- changes the characteristics of a population
- Distribution curve remains the same shape but the mean shifts to the left or right
STABILISING SELECTION
- Favours/selects phenotypes around the mean of a population
- preserves the characteristics of a population
- distribution curve becomes narrower and higher but the mean doesn’t change
A severe cold spell in 1996 killed over 50% of swallows living on cliffs in Nebraska. Biologists collected nearly 2000 dead swallows from beneath the cliffs and captured around 1000 living ones. By measuring the body mass of the birds, they found that birds with a larger than average body mass survived the cold spell better than ones with smaller than average body mass. State, giving your reasons, which type of selection was taking place here
Directional selection - because the birds to one side of the mean (heavier birds) were being selected for, while those to the other side of the mean (lighter birds) were being selected against. The population’s characteristics are being changed, not preserved
